To the beginning of the george zimmerman trial. He is charged in the shooting death of trayvon martin. Opening statements about to start today after a bombshell ruling in the defendant's favor this weekend. Abc's matt gutman has the latest. Joins us from sanford, florida, this morning. Good morning, matt. Reporter: Good morning, robin. Lawyers today are expected to take most of the day to make that crucial pitch to jurors. Nobody disputes that george zimmerman shot and killed trayvon martin. The question today is is it second degree murder or is it self-defense? This morning, opening statements 16 months in the making. The jury of six women will first hear the prosecution, accusing zimmerman of killing 17-year-old trayvon martin with, quote, a depraved mind. To prove second degree murder, prosecutors must prove that zimmerman was filled with ill will, hatred or spite. Reporter: The defense fighting to prove zimmerman seen in this police reenactment from last year acted in self-defense. Said, you got a problem now and then he was here and he punched me in the face. Reporter: Already over the weekend judge debra nelson making a pivotal ruling. On this 911 tape -- so you think he's yelling help? Yes. What is your -- preventing the prosecution from using two expert witnesses who say that voice you hear is martin pleading for his life. Without having those experts identify whose voice it was on the 911 tape the prosecution now has a hard time proving zimmerman followed someone, was the aggressor and gunned him down without just cause. Reporter: Experts hired by zimmerman say it's his voice calling for help 14 times in 40 seconds. All of it heaping more pressure on that six-person jury, five of them mothers ranking in age from THEIR 20s TO THEIR 60s. All of them will now be sequestered away from their families and news coverage and barraged with as many as 200 witnesses including neighbors who saw the scuffle that night and the last person who allegedly speak with martin, the unnamed friend known only as witness number eight. He said a man's following him again. Behind him. I say, run. Reporter: She saves she was on the phone with martin in the seconds before he died. Martin's mother says she'll be here throughout the trial. I'm committed that I have to be there for trayvon. Reporter: Now, on the other side of the courtroom from trayvon martin's mother will be an entire row of zimmerman family, his parents, likely his siblings and his friends and in the 16 months since that shooting we know that the families have not exchanged a single word. Robin. Matt, have you gotten a sense already what the strategy will be for both sides? Reporter: They've been outlining this for quite some time. The defense will try to show that trayvon martin in some way instigated a fight that led george zimmerman to shoot him saying he had no alternative. He felt like he had to save his life. The prosecution is going to say he knew what he was doing, george zimmerman went out there, he had a gun chamber, a bullet chambered in his gun, ready to shoot. He instigated the fight. He did not follow the orders of the police dispatcher and continued to follow martin anyway. They are going to say that it was zimmerman's fault throughout. Robin. All right, matt, thank you very much and as we've been saying the last should last two to four weeks.

This transcript has been automatically generated and may not be 100% accurate.

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